1. Field of the Invention
The present invention relates to a method and an apparatus for forming an image with multiple beams.
2. Description of the Related Art
In a typical electrophotographic image forming apparatus, a laser diode exposes a static electric charge formed on a photosensitive drum to a laser light, and thereby forming an electrostatic latent image. Then, a visible image is formed by developing the electrostatic latent image with a developer. A conventional laser diode generally emits one to four laser beams or at most eight laser beams per laser diode element. Recently, a surface emitting laser referred to as a vertical-cavity surface-emitting laser (VCSEL) has been commercially available, and put to practical use. Image forming apparatuses that include the VCSEL can form images at high resolution and high speed.
Japanese Patent Application Laid-open No. 2007-021826 discloses an image forming apparatus (an optical writing device) that employs a VCSEL. In this image forming apparatus, as shown in FIG. 22, a light source unit 1001 is composed of a laser diode array in which a plurality of light sources (a plurality of laser diodes) are arrayed in a lattice-like pattern or a surface emitting laser in which a plurality of light sources (a plurality of VCSELs or a plurality of surface-emitting laser diodes) are arrayed on the same chip in a lattice-like pattern. The layout and the orientation of the light source unit 1001 is adjusted in such a manner that an array direction of the light sources makes an angle θ with respect to a rotating shaft of a deflector such as a polygon mirror.
In the light source unit 1001 shown in FIG. 22, each of the light sources is denoted by a combination of a vertical array direction and a lateral array direction. The vertical array direction is separated into “a”, “b”, and “c” from the top in a vertical direction. The lateral array direction is separated into “1”, “2”, “3”, and “4” from the left in a lateral direction. For example, the top-left light source is denoted by “a1”. The light source unit 1001 is tilted at the angle θ, so that each of the light sources exposes a different scan field. It is assumed that two of the light sources compose one pixel. For example, the light sources a1 and a2 compose one pixel, and the light sources a3 and a4 compose another one pixel. Such pixels, which are respectively composed by the two light sources, are illustrated on the extreme right in FIG. 22. Furthermore, it is assumed that the vertical direction in FIG. 22 corresponds to a sub-scanning direction, and a center-to-center distance between the pixels is equivalent to 600 dots per inch (dpi). In other words, a center-to-center distance between each two of the light sources composing one pixel is equivalent to 1200 dpi. Namely, the light-source density is twice as much as the pixel density. By changing a light intensity ratio of the light sources composing one pixel, a position of a gravity center of the pixel can be shifted in the sub-scanning direction. Consequently, it is possible to form an image at high resolution.
The VCSEL can emit about forty laser beams from one chip, so that it can be considered that an image forming apparatus can form an image at high resolution and high speed by the use of the VCSEL. However, even if a laser diode is simply replaced with the VCSEL, it is not enough to form a latent image at sufficiently high resolution. For example, the VCSEL generates multiple laser beams, as if a surface were formed by the laser beams, from a predetermined light-emitting region, so that a light intensity of each of the laser beams to be emitted needs to be controlled to a target light intensity. Furthermore, the VCSEL has higher degree of integration of the laser beams, so that to form a latent image at high resolution stably, it is necessary to control the light intensity of each of the laser beams.
As the number of laser beams increases, it obviously takes a longer time to control a light intensity of each of the laser beams. Therefore, an image forming apparatus that employs the VCSEL is slower as compared with that that employs a laser diode. If the light-intensity control process is skipped to increase the speed, the resolution decreases.
To solve the problems, various technologies have been developed. For example, in the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2007-021826, the optical writing device includes a plurality of light-emitting elements and a light-emitting light source unit having a light-intensity detecting element. The light-intensity detecting element detects an intensity of a light emitted from each of the light-emitting elements. To control a light intensity of each of optical beams, the optical writing device further includes volume resistances respectively for each of the optical beams and a sample-and-hold capacitor. With the conventional technology disclosed in Japanese Patent Application Laid-open No. 2007-021826, it is possible to control light intensities of multiple laser beams. However, the size of a control circuit of the VCSEL is disadvantageously large. In addition, each of the volume resistances needs to be reset to adjust the light intensities on the number of times corresponding to the number of the laser beams to be emitted, so that the work efficiency lowers, and the maintenance frequency is increased.
Furthermore, in a conventional technology disclosed in Japanese Patent Application Laid-open No. 2005-161790, the light intensity is controlled as follows. A first measuring unit separates each of optical beams output from a light source into a first optical beam and a second optical beam, and measures a light intensity of the first optical beam. A light-intensity control unit controls the light intensity of the first optical beam measured by the first measuring unit to a light intensity indicated in a light-intensity signal. A light intensity of the second optical beam is measured. A light-intensity correction value of each of the optical beams for controlling the light intensity of the second optical beam to be substantially equalized among a plurality of the optical beams is obtained based on the light intensity of the second optical beam. The obtained light-intensity correction value is stored in a storing unit.
With the conventional technology disclosed in Japanese Patent Application Laid-open No. 2005-161790, it is also possible to control a light intensity of each of laser beams emitted from a VCSEL. If the number of the laser beams is not many, it is possible to form an image with controlling a light intensity of each of the laser beams at a sufficiently-high feedback speed. However, the VCSEL emits multiple laser beams. Therefore, due to an environmental fluctuation of the VCSEL, it may fail of a feedback with respect to a control of a light-intensity of each of the laser beams within a scanning time of an image forming process with a sufficiently-high efficiency. Furthermore, when laser diode elements composing the VCSEL cannot provide a predetermined light intensity with a correction value in a default-setting correction range, it is not possible to complete the image forming process with preventing a currently-formed image from being critically affected. In addition, it is not possible to correct the light intensity efficiently.
As described above, when a light intensity of each of laser beams emitted from a VCSEL is to be controlled, as the number of the laser beams increases, the number of control processes also increases. Therefore, it is not possible to sufficiently take advantages of the VCSEL, such as high resolution and high speed. In addition, the production cost and the maintenance cost disadvantageously increase. Thus, there is room for improvement.